How to unlock hundreds of secret 'Super Mario Bros.' levels hidden on the cartridge

Nintendo's 1985 classic video game "Super Mario Bros."Nintendo
The original "Super Mario Bros." is more than just the 32 or so levels you've played. The original "Super Mario Bros." actually has hundreds more levels. Hundreds!

Those levels, however, are messed up.

They're broken, contain enemies that have no place within them (a Goomba underwater?!), and look completely bananas. They're the bastard children of Nintendo's most revered game, hidden away in the annals of each cartridge's aging chips. These are levels Nintendo never intended anyone to see, and the company has never really spoken about them as a result.

But intrepid Nintendo enthusiasts have cracked the crazy secret process for unlocking them.

There's one well-known secret world in the original 'Super Mario Bros.' called the 'Negative World.'

It's just one of many of the hidden levels contained in "Super Mario Bros." and it's perhaps the most widely known. You can reach it by performing an awkward backwards jump in the game's second level, 1-2, like so:

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It's so-named because of the '-1' seen in the 'World' area of the screen:

Nintendo

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The level itself is just a standard underwater level, though it's impossible to progress beyond it. Exiting the level results in a loop.

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To access hundreds of other hidden levels in the original 'Super Mario Bros.' is far more complex. You need three items to pull it off. First you need an original 'Super Mario Bros.' cartridge:

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And a copy of 'Tennis' for the original Nintendo Entertainment System: